When pipes are laid undersea in shallow and medium depths, they are laid from floating supports fitted with handling means by assembling together unit lengths of tube that are generally 12 meters (m) or 24 m long. These lengths are handled while in the horizontal position and they are supplied to an assembly line where they are joined together end to end to form a continuous pipe which is displaced, generally in steps of 12 m or 24 m towards the stern of the barge. In the zone where it leaves the barge, the pipe is supported by a semi-floating structure of controlled curvature referred to as a “stinger” whose function is to limit the curvature of the pipe so as to prevent it from kinking. The new unit length is docked to the pipe that has already been made in the forward zone of the barge, at the first assembly station. This does not present great difficulty since the pipe resting on carriages or rollers can be maintained in extremely rigid manner, with welding then being performed on two parts that are almost perfectly stationary relative to each other, even in the event of the barge moving considerably because it is subjected to sea conditions.
In greater depths, and particularly for depths exceeding 1000 m or even 2000 m or more, laying is implemented in the so-called “J” configuration since the portion of pipe between the level of the ship and its point of contact on the sea bed takes up a J-shape. Such J-laying is described in particular in French patent applications Nos. 99/05534 and 99/11584. During laying, connecting together successive portions of pipe requires the portion of pipe that is already in the water to be kept stable, requires the top end of said portion of pipe which is above the water to be presented to the bottom end of the new portion of pipe, and requires the two ends of the pipe elements that are to be welded together to be held firmly together so that welding can be performed without risk of damage due to the various movements of the ship and of the portion of pipe that has already been made and that is suspended down to the bottom of the sea. The damage that is to be feared is constituted mainly by microcracking during the welding process which at best will lead to the weld being rejected during quality control, and as worst will lead to a break with the entire assembly then being lost while an installation is being put into service.
During such J-laying, the difficulty lies in the fact that all of the operations are performed at a single location, generally situated on the deck of the ship, and thus close to the bottom end of the tower, and in addition all of the operations need to be performed as quickly as possible because of the extremely high cost per hour of an installation ship.
The pipe that has already been made is held in suspension in the bottom portion of the tower and is guided at the bottom of said J-laying tower, being held securely by a system of external clamps over a length which cannot exceed several meters at most, for example as described in French patent application No. 99/14525. The portion of the pipe that is underwater is then subjected to bending which is transferred to its portion out of the water, whereas in contrast the new length of pipe or segment which generally measures 24 m to 48 m can be held securely within the J-laying tower. As a result, during movement of the ship due to the effects of swell, wind, and currents, the portion of pipe that has already been made and that is in suspension constitutes the least rigid portion in the zone close to the welding plane, in spite of being held very securely in the bottom portion of said tower.
When performing S-laying on a barge, it is general practice to use a centering insert fitted with pneumatic means for centering the two pipe portions. The centering insert is constituted by a system of internal clamps enabling the ends of the pipes that are to be welded end to end to be held facing each other and to cause their respective axes to coincide. In addition, when the thickness of the pipes is not too great, the forces generated against the wall by the clamps, more precisely by the pushers against the wall, have a tendency to return pipes that have become ovalized to a round configuration, thus greatly facilitating welding operations. Pipes often present manufacturing defects which give rise to a section that is of oval shape, it being understood that the difference in diameter between the smallest diameter and the largest diameter of a given section is of the order of a few millimeters, or about 1 centimeter (cm) or more. The simplest clamping systems perform centering solely in the vicinity of the welding plane and are said to be “two-plane” centering systems, the two planes being on either side of the welding plane. These planes are substantially perpendicular to the axis of the pipe elements after welding. More sophisticated clamping systems have clamps disposed in four planes, i.e. two additional planes situated on opposite sides of the welding plane so as to improve the manner in which the two pipe elements are brought into alignment, and causing the clamp to be engaged in each of the pipe elements on either side of the welding plane. Such centering inserts are generally fitted in their central portion with an anvil, in particular as described in French patent application No. 99/15254, which anvil is applied against the inside face of the pipe facing the welding zone, and which has the function of maintaining the weld bath during welding.
Thus, a centering insert device seeks to hold the ends of two pipe elements that are to be joined together end to end firmly in such a manner that all degrees of freedom in translation and in rotation are blocked, so as to avoid any parasitic relative movements between the two faces to be welded together during the welding operations proper, this stage remaining the most difficult stage during installation of undersea pipes in deep and ultra-deep water, i.e. in depths in the range 1500 m or 2000 m, or indeed 3000 m to 4000 m, or more.
A centering insert device serves to become securely engaged in each of the two pipe elements to be joined together end to end and to provide maximum stiffness in the vicinity of the junction plane so as to minimize relative movements between the two faces to be welded together, particularly when automatic welding is implemented. Above all, a centering insert device serves mainly to minimize rotation about axes contained in the section plane of the pipe.
In prior embodiments such as those described in U.S. Pat. Nos. 3,937,382, 4,418,860, and GB 1 283 922, each of the planes of said two- or four-plane centering inserts comprises clamping systems of the type presenting a circular conforming effect, i.e. they tend to return pipes of small or medium thickness to a “round” shape while also performing their main purpose of centering.
More precisely, in the present description, the term “clamping system with a circular conforming effect” is used to mean a plurality of clamps in a common centering plane, each clamp comprising a thrust block and a thruster device such as an actuator actuating said thrust block, said thrust blocks being disposed substantially radially about said longitudinal axis of said pipe and co-operating in such a manner that by applying thrust to an inside wall of oval section, they can deform it so as to return it to a “round” shape, i.e. circularize it in order to obtain a circular section.
A clamping system necessarily includes a minimum of three thrusters, and preferably has four, six, eight, twelve or more thrusters, the higher the number of thrusters the better the circularizing. The number of thrusters is generally determined by the space available for installing them within the main body of the clamping system.
Clamping systems with a circular conforming effect in the prior art comprise in particular a first pair of opposite thrust blocks which come into contact with the opposite points that are the closest together of the oval section of an oval-section pipe and which continue to apply thrust, thereby deforming the pipe so as to circularize it until a second opposite pair of thrust blocks come into abutment against opposite points of the oval section wall that were initially furthest apart, such that all of the thrust blocks in a given section plane can be tightened and blocked against the wall.
Those clamp systems comprise a main longitudinal body supporting a plurality of clamps each comprising a respective thrust block that comes into abutment against the inside wall of the pipe, together with a thruster device, in general actuators, serving to actuate said thrust blocks and move them against said inside wall. The thrusters can move in guide housings provided in the main body. Said thrust blocks move in translation in respective radial directions relative to the longitudinal axis of the pipe and of said main body, i.e. in directions perpendicular to said longitudinal axis.
In order to achieve such engagement, the centering insert devices described in the prior art have at least four section planes, each containing circular conforming effect clamps.
In the present description, the term “centering plane” or “section plane” is used to mean a plane substantially perpendicular to the longitudinal axis of the pipe or of said main body, as appropriate.
The inventors have observed that the problem which arises during J-laying when using clamping systems with a circular conforming effect, is that the main body of the centering insert is subjected to very high levels of stress while the pipe elements are being brought into alignment, and that can cause said body to bend, which means that the structure of said main body needs to be overdimensioned in order to be able to deliver the desired stabilization service during the stage of welding together the ends of said pipe elements.